Magnesium, which is an active light metal, has a diagonal relationship with lithium, has a similar ionic radius to that of lithium, and also has similar chemical properties to those of lithium. Since a metallic magnesium negative electrode has a relatively high theoretical specific capacity (2205 mAh/g), is inexpensive and high in melting point (649° C.), is easily processed, and is also environmentally-friendly, a magnesium secondary battery in which magnesium is used for a negative electrode is increasingly attracting attention (see Non-Patent Documents 1 to 3). A magnesium secondary battery is superior in terms of cost and energy density to a lithium ion cell and thus is a promising eco-friendly battery and is expected to serve as a high-capacity power battery system for the next generation (see Non-Patent Documents 4 and 5).
Conventionally, the development of a magnesium secondary battery has been restricted by two factors. The first factor is as follows: while both of a magnesium ion and a lithium ion have a small ionic radius, a magnesium ion has a higher charge and undergoes a strong solvation action, and therefore no or few substrates are available that allow the insertion of magnesium ions, thus limiting the selection of positive electrode materials. The second factor is as follows: magnesium forms a passive film in most electrolytic solutions, and this passive film is a poor conductor for Mg2+ ions. Therefore, it is difficult to prepare a magnesium secondary battery electrolytic solution system having a high reversible deposition-dissolution rate of magnesium and a wide electrochemical window, inhibiting the development of a magnesium secondary battery.
A magnesium electrode forms no passive film in a Grignard reagent/ether solution and is also excellent in reversible deposition properties of magnesium, but a common Grignard reagent, of which the stable electrochemical window is narrow, cannot be easily applied to a magnesium secondary battery (see Non-Patent Documents 6 and 7). In 2000, Nature journal disclosed a novel electrolytic solution system that can be applied to a magnesium secondary battery, researched and developed by Aurbach (Israeli scientist) et al. (see Non-Patent Document 8), and disclosed that magnesium can be reversibly deposited from an ether solution of Mg(AX4-nRn)2, where A=Al, B, As, P, Sb or the like, X=Cl or Br, R represents an alkyl group, 0<n<4, and n′+n″=n. This novel formulation is considered to have been obtained by the reaction of a Lewis acid RR′Mg with a Lewis salt AX3-nRn′R′n″, and was named as “first generation electrolytic solution” by the D. Aurbach team. This system has a significantly enhanced conductivity in a 0.3 to 0.5 M electrolyte solution at room temperature and has a significantly improved anodic stability as compared with the case of a Grignard reagent. At the end of year 2007, the Aurbach team succeeded in synthesis of a magnesium-aluminum-halogen complex, in which all ligands are phenyl groups, in an electrolytic THF solution system and such an electrolytic solution system was referred to as “second generation electrolytic solution” (see Non-Patent Document 9). The electrolytic solution was obtained by reacting PhMgCl as a Lewis salt and AlCl3 as a Lewis acid in a ratio of 2:1 in tetrahydrofuran solvent, and the electrochemical window thereof was 3 V or more, the overpotential thereof in magnesium deposition was less than 0.2 V, and the conductivity of the solution was clearly enhanced as compared with conventional systems. The Grignard reagent, PhMgCl, in the electrolytic solution, however, is highly reductive and must be measured and stored in conditions of no moisture and isolation from air, and thus the range of application for the system is limited.    Non-Patent Document 1: Gregory T D, Hoffman R J, Winterton R C, Development of an ambient secondary magnesium cell, Journal of the Electrochemical Society, 1990, 137(3): 775-780    Non-Patent Document 2: Besenhard J O, Winter M, Advances in cell technology: rechargeable magnesium cells and novel negative electrode materials for lithium ion cells, Chemphyschem, 2002, 3(2): 155-159    Non-Patent Document 3: Levi E, Gofer Y, Aurbach D, On the Way to Rechargeable Mg Cells: The Challenge of New Cathode Materials, Chemistry of Materials, 2010, 22(3): 860-868    Non-Patent Document 4: Lossius L P, Emmenegger F, Plating of magnesium from organic solvents, Electrochimica Acta, 1996, 41(3): 445-447    Non-Patent Document 5: Aurbach D, Gofer Y, Lu Z, et al. A comparison between the electrochemical behavior of reversible magnesium and lithium electrodes, Journal of Power Sources, 2001, 97(8): 269-273    Non-Patent Document 6: Liebenow C, Yang Z, Lobitz P, The electrodeposition of magnesium using solutions of organomagnesium halides, amidomagnesium halides and magnesium organoborates, Electrochemistry Communications, 2000, 2(9): 641-645    Non-Patent Document 7: Guo Y S, Yang J, NuLi Y N, Study of electronic effect of Grignard reagents on their electrochemical behavior, Electrochemistry Communications, 2010, 12(2): 1671-1673    Non-Patent Document 8: Aurbach D, Lu Z, Schechter A, Prototype system for rechargeable magnesium cells, Nature, 2000, 407(6805): 724-727    Non-Patent Document 9: Oren Mizrahi, Nir Amir, Aurbach D, Electrolyte Solutions with a Wide Electrochemical Window for Rechargeable Magnesium Cells, Journal of The Electrochemical Society, 2007, 155(2); A103-A109